New Member/First Rebuild/Zero Compression... lol

[Reverend]

Hi folks!

So, I wanted to say hey and that I'm glad to have found this resource! I've been reading my butt off the last week or so and this place is full of great info!

So, I don't want to overwhelm y'all, so I'll keep my question short.

I have a 1978 XS400 2e. I am hoping to turn it into a reliable cafe racer for a friend.

I have it some experience on bikes, having kept several bikes on the road and bringing a few back from the dead.

My diagnostic skills suck, but I can follow a manual pretty well, and have access to a halfway decent workshop.

So, the first thing I've been reading up on is what to do if the bike has no compression. At first I thought this was a death sentence, but the more I read, the more it seems that this is a handle-able situation.

If I may, what is the first thing to check? I gather that Harbor Freight has a decent compression gauge, but I don't really know how to diagnose exactly what is leaking.

Can anyone give me a hint as to what to check first? Is there anything that y'all have noticed is a chronic problem on these bikes?

Thanks! Lots of questions to follow!

 

[BBS360]

What evactly do you mean by "no compression"?

Get a pressure gauge. Check both sides, wide open throttle, record psi.
Drop an oz of oil in the sparkplug hole and try again, both sides, record.

If the compression goes up significantly with oil that indicates that you are losing compression around the piston rings.
If not then you're losing it through the valves.

How bad the compression is and where itsi happening will determine next steps.
Could be a lost cause.
Could need new rings, a honing, both, oversize rings, or maybe valve honing, cutting, replacing, or more.

 

[Reverend]

Can a compression check be performed even though the bike won’t start yet?

 

[motoTrooper]

Yes, you're only measuring how tight the tolerances are inside the combustion chamber and if the throttle is closed it restricts how much air can go into the chamber. Make sure the fuel is shut off at the petcock (it's a good habit to shut it off whenever the bike is not in use anyway). Pull out both sparkplugs to ease cranking and make sure no loose dirt or debris can get sucked into the holes. It's easier/quicker to measure if you can use the electric starter but kickstarting the engine can work too.

 

[amc49]

Best way is to use a car battery (do NOT have the car running!) so you can get multiple readings. I get at least 3-4 from each cylinder. Make sure you FULLY release the gauge air every time before spinning again. Harbor Freight gauge may work or be garbage, Electric starter is best but let it cool at use over 15 seconds. Crank each time until the gauge needle quits rising and note any difference in how fast one vs. the other cylinder rises too.

The shorter the gauge hose is the better on small displacement cylinders like bike. The area tilts the numbers. If the release valve has gone bad in gauge you absolutely CANNOT use a tire one either, compression gauge valves are wltra-low sprung to read any small pressure at all.

 

[Reverend]

So... I think I need to back up... lol.

Here's the bike I'm working on.

I have approximately 2.4 million questions, and I'm researching a ton...

I have a 3k budget for this build.

I'm picking up a compression gauge tomorrow and will run that check asap.

First Questions, in no particular order, just a question dump.

Is it possible/desirable to update this with electronic ignition, and completely updated electrical systems in order to get rid of as much 1978 tech as possible? It would feel great to tear all this wiring out and start fresh!

I've never ridden a bike with drum brakes before. How finicky are they to rebuild? Are there bikes out there with wheels with disc brakes that I could switch over easily?

It's late and my head is spinning, so I'm off to bed. Been thinking about this for weeks, trying to plan. I think it's just time to start turning wrenches.

Thanks guys.

Rev

 

[Tyler Shiells]

If you end up having poor compression and need a rebore, over size rings and pistons, expect to pay in excess of $500 just for that job alone. That's if your doing all the work and paying someone to machine the cylinder jugs. Over sizes pistons are fairly rare. NOS is out there but not plentiful. Someone in Germany makes over sized rings but they are pricey.
I just went through that whole process on my build. If your interested check out my thread. It saved my engine. I'm over 150psi each cylinder now. Brand new again.
Good luck.

 

[amc49]

You can do a compression test with no carbs on engine, as that one seems to be.

The biggest problem with a rebore is not the money so much as one of quality work. ALL the shops will tell you they can do it but few can correctly. Boring now is fast becoming a lost art. I myself fear needing it done, certainly nobody here in the D/FW area can do it to my knowledge even though several places would swear they can. The last time I heard that from a well known local hot rod bike shop they gave me junk back after scrapping unreplaceable parts.

 

[amc49]

Drum brakes are easy but will not stop as well as disc, but disc on back is pretty much worthless as you lock them too easy when all the weight goes to the front. I'd leave back alone and double disc the front, can't go wrong there.

Your front brake there is double leading shoe and not hard to adjust but BOTH pads need to contact at the same time to get max stopping out of one. What that link on the front assembly does.

 

[BBS360]

There are less electronic ignition options now that pamco has packed up.
I think MikesXS has a chinese knockoff that can work but I seem to recall reading about issues there already.

Newtronics has one too.

The 1978 tech really isn't that bad. Ignition coils are still ignition coils.

 

[Reverend]

Drum brakes are easy but will not stop as well as disc, but disc on back is pretty much worthless as you lock them too easy when all the weight goes to the front. I'd leave back alone and double disc the front, can't go wrong there.

Your front brake there is double leading shoe and not hard to adjust but BOTH pads need to contact at the same time to get max stopping out of one. What that link on the front assembly does.

I might be looking to do exactly what you suggest. Find a compatible wheel upfront with a disc brake and match it to the back wheel. Thanks!